Preheat a grill for medium heat (350° to 450°). Rub pork with 1/2 tsp. oil. In a bowl, whisk together zest, juice, honey, and 3/4 tsp. salt and divide between 2 bowls.
Grill lemon, turning occasionally, until very soft, about 8 minutes, and grill pork, turning occasionally and brushing with half the lime glaze, until a thermometer inserted in the thickest part reaches 145°, about 20 minutes.
Let lemon cool, remove seeds and central white core, and chop finely. Transfer pork to a platter, tent with foil, and let rest about 10 minutes. Meanwhile, boil remaining glaze in a small saucepan, stirring occasionally, until thickened, about 5 minutes. Set aside.
In a bowl, combine remaining 2 tsp. oil, grapefruits, oranges, thyme, remaining 1/2 tsp. salt, chopped lemon, and pistachios.
Slice pork, brush with thickened glaze, and serve with citrus salsa.
Note: Nutritional analysis is per serving.